I. Field of the Invention
The present invention relates generally to antennas, and more particularly, to dual strip multiple frequency antennas with each strip implemented as a periodic mesh pattern. The invention further relates to internal antennas for wireless devices, especially having reduced size, improved bandwidth and improved radiation characteristics. The operation of a dual strip antenna is described in the patent application DUAL STRIP ANTENNA, Ser. No. 09/090,478, assigned to the assignee of the present invention, of which the disclosure is incorporated by reference herein.
II. Description of the Related Art
Antennas are an important component of wireless communication devices and systems. Although antennas are available in numerous different shapes and sizes, they each operate according to the same basic electromagnetic principles. An antenna is a structure associated with a region of transition between a guided wave and a free-space wave, or vice versa. As a general principle, a guided wave traveling along a transmission line which opens out will radiate as a free-space wave, also known as an electromagnetic wave.
In recent years, with an increase in use of personal wireless communication devices, such as hand-held and mobile cellular and personal communication services (PCS) phones, the need for suitable small antennas for such communication devices has increased. Recent developments in integrated circuits and battery technology have enabled the size and weight of such communication devices to be reduced drastically over the past several years. One area in which a reduction in size is still desired is communication device antennas. This is due to the fact that the size of the antenna can play an important role in decreasing the size of the device. In addition, the antenna size and shape impacts device aesthetics and manufacturing costs.
One important factor to consider in designing antennas for wireless communication devices is the antenna radiation pattern. In a typical application, the communication device must be able to communicate with another such device or a base station, hub, or satellite which can be located in any number of directions from the device. Consequently, it is essential that the antennas for such wireless communication devices have an approximately omnidirectional radiation pattern.
Another important factor to be considered in designing antennas for wireless communication devices is the antenna""s bandwidth. For example, wireless devices such as phones used with PCS communication systems operate over a frequency band of 1.85-1.99 GHz, thus, requiring a useful bandwidth of 7.29 percent. A phone for use with typical cellular communication systems operates over a frequency band of 824-894 MHz, which requires a bandwidth of 8.14 percent. Accordingly, antennas for use on these types of wireless communication devices must be designed to meet the appropriate bandwidth requirements, or communication signals are severely attenuated.
One type of antenna commonly used in wireless communication devices is the whip antenna, which is easily retracted into the device when not in use. There are, however, several disadvantages associated with the whip antenna. Often, the whip antenna is subject to damage by catching on objects, people, or surfaces when extended for use, or even when retracted. Even when the whip antenna is designed to be retractable in order to prevent such damage, it can extend across an entire dimension of the device and interfere with placement of advanced features and circuits within some portions of the device. It may also require a minimum device housing dimension when retracted that is larger than desired. While the antenna can be configured with additional telescoping sections to reduce size when retracted, it would generally be perceived as less aesthetic, more flimsy or unstable, or less operational by consumers.
Furthermore, a whip antenna has a radiation pattern that is toroidal in nature, that is, shaped like a donut, with a null at the center. When a cellular phone or other wireless device using such an antenna is held with the antenna perpendicular to the ground, at a 90 degree angle to the ground or local horizontal plane, this null has a central axis that is also inclined at a 90 degree angle. This generally does not prevent reception of signals, because incoming signals are not constrained to arrive at a 90 degree angle relative to the antenna. However, phone users frequently tilt their cellular phones during use, causing any associated whip antenna to be tilted as well. It has been observed that cellular phone users typically tilt their phones at around a 60 degree angle relative to the local horizon (30 degrees from vertical), causing the whip antenna to be inclined at a 60 degree angle. This results in the null central axis also being oriented at a 60 degree angle. At that angle, the null prevents reception of incoming signals arriving at a 60 degree angle. Unfortunately, incoming signals in cellular communication systems often arrive at angles around or in the range of 60 degrees, and there is an increasing likelihood that the mis-oriented null will prevent reception of some signals.
Another type of antenna which might appear suitable for use in wireless communication devices is a conformal antenna. Generally, conformal antennas follow the shape of the surface on which they are mounted and generally exhibit a very low profile. There are several different types of conformal antennas, such as patch, microstrip, and stripline antennas. Microstrip antennas, in particular, have recently been used in personal communication devices.
As the term suggests, a microstrip antenna includes a patch or a microstrip element, which is also commonly referred to as a radiator patch. The length of the microstrip element is set in relation to the wavelength xcex0 associated with a resonant frequency f0, which is selected to match the frequency of interest, such as 800 MHz or 1900 MHz. Commonly used lengths of microstrip elements are half wavelength (xcex0/2) and quarter wavelength (xcex0/4). Although, a few types of microstrip antennas have recently been used in wireless communication devices, further improvement is desired in several areas. One such area in which a further improvement is desired is a reduction in overall size. Another area in which significant improvement is required is in bandwidth. Current patch or microstrip antenna designs do not appear to obtain the desired 7.29 to 8.14 percent or more bandwidth characteristics desired for use in advanced communication systems, in a practical size.
Therefore, a new antenna structure and technique for manufacturing antennas are needed to achieve bandwidths more commensurate with advanced communication system demands. In addition, the antenna structure should be conducive to internal mounting to provide more flexible component positioning within the wireless device, greatly improved aesthetics, and decreased antenna damage.
The present invention is directed to a dual strip antenna. According to the present invention, the dual strip antenna includes a first and a second strip, each made of a conductive material, such as a metallic plate. The first and second strips are separated by a dielectric material. The first strip is electrically connected to the second strip at one end.
A coaxial feed structure is connected or coupled to the dual strip antenna. In a preferred embodiment, a positive terminal of the coaxial feed is electrically connected to the first strip, and a negative terminal of the coaxial feed is electrically connected to the second strip. In another embodiment, these terminals or polarities are reversed.
The dual strip antenna can also be constructed by depositing one or more layers of conductive material such as metallic compounds, conductive resins, or conductive ceramics in the form of strips on two sides of a dielectric substrate. In this technique, one end of each of the strips is electrically connected together. This electrical connection can be implemented by a variety of means, such as conductive wires, solder materials, conductive tapes, conductive compounds or one or more plated through vias. The substrate provides a desired shape or relative positioning for the strips deposited thereon.
In one embodiment, the first and second strips are positioned approximately parallel to one another, as in two parallel planes.
The dual strip antenna provides an increase in bandwidth over typical quarter wavelength or half wavelength patch antennas. Experimental results have shown that the dual strip antenna has a bandwidth of at least approximately 10 percent, which is very advantageous for use with wireless devices such as cellular and PCS telephones.
Implementing the strips in periodic mesh patterns instead of continuous conductive strips can further reduce the size of the dual strip antenna. A periodic mesh pattern can be used for one or both strips. Additionally, the periodic mesh pattern may be used for only a portion of a strip. The periodic mesh may be a brick wall pattern or honeycomb pattern or any repetitive pattern.
Implementing a strip as a periodic mesh pattern increases the distance traversed by the current on the strip. The distance is increased because the current must traverse deviations along the mesh pattern rather than a straight line path available on a continuous conductor.
The radiation pattern of the antenna is not affected by the implementation of the strips as periodic mesh patterns. The dimensions of the periodic mesh are much smaller than the dimensions of the strip. The dimensions of the strip are in turn smaller than the wavelength of the broadcast frequency band. The current and voltage potentials along the periodic mesh differ only a slight amount from the current and voltage potentials along a continuous conductive strip. Thus the far field radiation pattern is the same using the periodic mesh pattern.